1. Field of the Invention
The present invention relates to a correlation detection apparatus used in detecting the correlation of a spread spectrum signal in a CDMA (Code Division Multiple Access) radio communication apparatus, and more particularly, to a correlation detection apparatus applicable to a digital matched filter.
2. Related Art
A matched filter is used at a receiving station to demodulate a spread spectrum signal that is generated by multiplying transmission data with a spreading code at a transmitting station.
FIG. 1 illustrates an example of a configuration of a conventional matched filter. The matched filter illustrated in FIG. 1 is a digital matched filter with 4 taps for spreading ratio 4 composed of FIR digital filters.
Digital signal IO is input to signal input terminal 1 of the digital matched filter. Digital signal IO is, for example, a digital signal generated by sampling a spread spectrum signal at a sampling frequency of 4,096 MHz. This digital signal IO is input to first flip-flop group 11 of shift register with taps 10 (hereinafter referred to as shift register 10), and then shifted sequentially to fourth flip-flop group 14 from first flip-flop group 11 in synchronization with clock CLK of 4,096 MHz input to clock input terminal 2.
Output Signals output from every tap and the flip-flop at the last stage of shift register 10 are respectively input to first to fourth multipliers 15 to 18. Corresponding despreading code C among from despreading code sequence C3C2C1C0 is provided to each of first to fourth multipliers 15 to 18. Accordingly, first multiplier 15 executes multiplication of an output signal from first flip-flop group 11 by despreading code Co (1 bit). Similarly, second to fourth multipliers 16 to 18 execute respectively multiplication of output signals from second to fourth flip-flop groups 12 to 14 respectively by despreading codes C1 to C3. First to fourth multiplies 15 to 18 respectively execute multiplication of respective output signals from first to fourth flip-flop groups by 1 when the despreading code indicates 0, and multiplication of output signals from first to fourth flip-flop groups by xe2x88x921 when the despreading code indicates 1.
First adder 19 adds outputs from first and second multipliers 15 and 16, second adder 20 adds outputs from third and fourth adders 17 and 18, and third multipliers 21 adds outputs from first and second adders 19 and 20. As a result, correlation value MFOUT of digital signal IO with despreading code sequence C3C2C1C0 is output to external through output terminal 3.
Conventionally, the digital matched filter described above rewrites despreading code sequence C3C2C1C0 on a basis of one symbol period to detect the correlation of each symbol. As a result, it is not possible to detect the correlation of delayed waves input after one symbol period passes.
Such a condition has remained the inconveniences that it is not possible to effectively utilize delayed waves input after one symbol period passes in the CDMA system that enables a plurality of delayed waves to be utilized. One of means of solving the inconveniences suggests a configuration having a plurality of parallel connected matched filters each of which is illustrated in FIG. 1. FIG. 2 illustrates the configuration having a plurality of parallel connected matched filters each of which is illustrated in FIG. 1. First to fourth digital matched filters M1 to M4 are provided in parallel.
However, the configuration with the plurality of parallel connected matched filters results in the problem that a circuit scale and power consumption are increased.
An object of the present invention is to provide a correlation detection apparatus capable of detecting the correlation of delayed waves arriving after one symbol period passes, and of decreasing a circuit scale and power consumption.
The correlation detection apparatus has a data storage section for storing symbol data, a plurality of correlation calculators connected to the data storage section in parallel, each for executing the correlation detection of the symbol data with a spreading code for despreading, and a code controller for rewriting the spreading code for each of the correlation calculators on a basis of a plurality of symbol periods individually.
According to the configuration, a plurality of correlation calculators are connected to the data storage section in parallel, and the spreading codes in respective correlation calculators are individually rewritten after a period corresponding to a plurality of symbols passes. Therefore, it is possible to detect the correlation of delayed waves arriving after one symbol period passes, and to decrease a circuit scale and power consumption largely compared with the case of arranging a plurality of digital matched filters in parallel to perform the processing.